As knifegirl observed above, Easter Sunday varies from year to year. In 1800, the mathematician Karl Friedrich Gauss devised the following algorithm to determine the date of Easter Sunday (for Western churches) for a given year.
- Let y be the year for which you wish to find Easter Sunday.
- Divide y by 19 and call the remainder a. (ignore the quotient)
- Divide y by 100 to obtain a quotient b and a remainder c.
- Divide b by 4 to get a quotient d and a remainder e.
- Calculate 8*b + 13. Then divide by 25 to get a quotient g. (ignore the remainder)
- Calculate 19*a +b -d -g +15. Then divide by 30 to get a remainder h. (ignore the quotient)
- Divide c by 4 to get a quotient j and a remainder k.
- Calculate a + 11*h. Divide by 319 to get a quotient m. (ignore the remainder)
- Calculate 2*e + 2*j -k -h +m +32. Divide by 7 to get a remainder r. (ignore the quotient)
- Calculate h -m +r +90. Divide by 25 to obtain a quotient n. (ignore the remainder)
- Calculate h -m +r +n +19. Divide by 32 to get a remainder p. (ignore the quotient)
Then, Easter Sunday for year y falls on day p of month n.
Obviously, this is somewhat tedious to do by hand each time, so if you're just interested in the destination rather than the journey I wrote a (probably very bad and largely untested) java applet to do this which can be found at http://www.bath.ac.uk/~ma2gdt/easter.html . | 
